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redundancy via DNS

we have several servers colocated with several ISP's
i am trying to sort out some configuration that would ensure good uptime for

i want to place the html documents of every customer on two separate servers
connected to separate ISP's
the dns servers will point to one server and the second one will be just a
backup, in case the main server goes down we just change the DNS and point
the affected domains to the backup server. when the main server is back up
the dns changes back to normal

and now my questions:
1. what should the times in zone files be set to to enable the dns change to
be propagated very quickly, say 5 minutes max.
   is it possible/wise to use TTL=0

2. if a domain has 2 name servers set during registration, are both of these
servers used for lookups? Or is it so that just the primary is querried if
it works, and the secondary is querried only if the primary is not

3. is this whole idea worth consideration anyway or should I forget it?

thanks for answers

Martin Dragun

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